The present invention relates to processes for recovering metals absorbed on a support of the resin type, in particular in the field of treatment and recycling of industrial effluents, in particular effluents resulting from photographic development, and concerns a process for separating and recovering metals absorbed on an ion-exchange resin and to a process and an installation for the treatment and recycling of at least some effluents of the aforementioned type, employing the aforementioned separation process.
At present, cationic resins saturated by metal ions or a mixture of metal ions are generally regenerated by one of the following processes:
elution by an acid or a chelating agent with or without a chromatographic effect,
regeneration in an electrolytic cell
pumping of the ions by means of resin fibres and under the influence of an electrostatic field toward compartments which receive the concentrated saline solutions.
Now all the processes described hereinbefore yield concentrated metallic salt solutions which are generally mixed with one another and with other components or are optionally monometallic as the result of an expensive treatment necessitating significant investment costs.
Furthermore, the aforementioned processes are limited with regard to the obtaining of metals to those of which the normal oxidation-reduction potential is higher than or equal to xe2x88x921.2 volts.
Moreover, various techniques are also currently known for treating these effluents, the most commonly used being precipitation, cementation, ion exchange, electrolysis, ultrafiltration and reverse osmosis, each of which is applied directly and independently of the other methods to the effluents to be treated, in particular to the bleaching/fixing baths loaded with silver.
The method of precipitation involves precipitating the silver ion using a sodium or potassium sulphide according to the equation:
2Ag++Sxe2x88x92xe2x88x92xe2x86x92Ag2S
This process definitively destroys the bath and produces silver sulphide which has to be recycled in silver metallurgy. This is achieved by carrying out xe2x80x9ccupellationxe2x80x9d during which the Ag is displaced by molten lead according to the reaction:
Ag2S+Pbxe2x86x922Ag+PbS
The lead is recovered by an extremely laborious complex thermal process commonly known as the xe2x80x9cPattinson processxe2x80x9d. This method is extremely toxic and problematic owing to the resultant environmental pollution.
For its part, cementation involves displacing the silver in solution with iron, which is electropositive, as follows
2Ag++Fexe2x86x922Ag+Fe++
This process is carried out by passing the solution to be desilvered into a cartridge containing iron or steel filings and is used mainly to desilver film washing water having a low silver content. A poor quality silver mixed with iron is obtained and the treated effluent is loaded with iron ions.
The ion-exchange method involves fixing the silver dithiosulphate complex present in the baths on an ion-exchange resin of the strongly basic type by exchanging silver dithiosulphate ions for chloride ions as follows:
3[[R]+Clxe2x88x92]+[Ag(S2O3) 2]xe2x88x92xe2x86x923[R]+[Ag(S2O3)2]xe2x88x92+3Clxe2x88x92
The silver is then eluted with a sodium thiosulphate or sodium chloride solution, and this eluate can be recycled in an electrolytic desilvering unit.
This process is particularly suitable for the treatment of film washing water or for finishing the desilvering of effluents already pre-desilvered by electrolysis and having low silver contents.
The main drawbacks of this process, as carried out at present are that:
the residual Ag content of the treated effluents often has a high dispersion, depending on the work regime of the development workshops, and the maximum levels of concentration in waste established by laws and regulations cannot always be guaranteed;
the effluents are loaded with chloride in proportion with the initial Ag content;
effluents originating from colour photographic processes containing ferric complexes such as iron III EDTA or ferrocyanide are also fixed and progressively poison the resin;
the anionic resins used cannot be recovered or reused;
the silver is not decomplexed relative to the thiosulphate and the treated bath never has a composition allowing it to be reused.
The electrolytic process involves depositing the Ag contained in the waste fixing or bleaching and fixing baths by direct electrolysis of these baths as follows:
Ag[(S2O3)2]xe2x88x92⇄Agxe2x88x92+2S2O3xe2x88x92
Ag++exe2x88x92xe2x86x92Ag
This electrolytic process can be carried out according to different variations or applications mentioned hereinafter:
1) The electrolysis of fixing or bleaching and fixing baths with regeneration thereof involves carrying out partial desilvering to a minimum content of 1 g/l under a voltage limited to 1.3 V. Under these moderate electrolysis conditions, the deposited Ag is of good quality, the current efficiency is almost 100% and the bath is not decomposed and is recycled after addition of xe2x80x9cregeneratingxe2x80x9d products which compensate the consumption of reagents by the irreversible reactions of the photographic process.
The volume of the bath increases as regeneration takes place, and the partially desilvered bath which is to be treated by a depolluting agent is periodically purged.
Furthermore, in the case of bleaching and fixing baths and owing to the presence of the Fe III EDTA complex in colour photographic processes, secondary reactions at the electrodes reduce the current efficiency.
In the best case, the recovery rate of the bath cannot exceed 60% as it is determined by the ratio between the average content at the inlet which is 2.5 g/l of Ag on average and that at the outlet limited by the process itself to 1 g/l.
The last evolutions of this method of direct electrolysis of the baths leads to desilvering with a minimal residual content of 0.5 g/l (still representing 20% of the initial content), produce waste in the form of graphite beads and are suitable only for the treatment of black and white photographic baths.
2) The electrolysis of fixing or bleaching baths leading to the destruction thereof ends with advanced desilvering (1 to 10 mg/l) by vigorous electrolysis at about 1.8 V. These vigorous conditions cause precipitation of Ag sulphide by decomposition of the thiosulphate. Precipitated Ag2S should be filtered after flocculation, and the decomposition of the thiosulphates also causes a release of toxic foul-smelling H2S. Furthermore, Ag2S has to be treated again by the above-described method of precipitation.
3) Electrolytic desilvering of the washing water involves carrying out electrolysis in special apparatuses comprising a cathode consisting either of a stainless steel foam or of a stack of vibrated graphite beads having a very large active surface area, the anode being made of titanium. These devices, which yield silver contents  less than 1 mg/l in the treated water, are also employed by depolluters in order to finish the desilvering of fixing or bleaching and fixing baths treated as described hereinbefore (in this case, the first desilvering operation by conventional electrolysis is limited to a content of 100 mg/l for economic reasons).
This process has the drawback of producing silver-covered graphite beads which, in turn, have to be treated.
Ultrafiltration and reverse osmosis are membrane-type processes employing modules containing semipermeable membranes of which the porosity is selected according to the substances to be eliminated. They necessitate relatively high working pressures and are suitable only for sparingly loaded water such as rinsing water, for example. Furthermore, their use necessitates very large, expensive technical means.
Thus, the first object of the present invention is to design a process for the separation and recovery of metals absorbed on an ion-exchange resin, for example after treatment of loaded effluents or of galvanoplastic baths not having the drawbacks or the limitations of currently known processes and, in particular, allowing different metals (for example, Au, Cu, Ag) saturating a, for example, cationic resin to be eluted selectively and to be recovered successively, in the order of increasing electronegativity or electropositivity, while not producing residues, in particular, of the heavy metal hydroxide type.
A further object of the invention is to provide a process and an installation for the treatment and recycling of at least some effluents resulting from photographic development with recovery of the silver present in the effluents, not having the drawbacks or the limitations of current processes, being able to treat sparingly loaded and highly loaded effluents equally well, yielding regenerated solutions which can be reused in photographic development in a quantity equivalent to the treated effluents, allowing almost all of the silver to be recovered and rejecting only minimal or almost zero quantities of final effluents.
Moreover, the process according to the invention should necessitate only limited investment and allow repeated reuse of the active substances involved in the treatment.
To this end, the invention relates firstly to a process for separating metal ions absorbed on an ion-exchange resin, characterised in that it involves suspending or percolating the preferably cationic resin loaded with a polar solvent containing an eluting agent or a mixture of eluting agents containing at least one organic or mineral ligand capable of complexing the metal ions to be separated, circulating a direct electric current, whether constant or not, in the eluting solution loaded with metal complexes or in the loaded mixture of resin and eluting solution forming an electrolyte between at least one cathode made of a metal identical to that of the metal ions to be recovered by deposition and at least one anode made of a material which is inert under the given physico-chemical conditions while keeping the cathode at an electric potential of which the value is slightly lower than the oxidation-reduction potential of the metal ion to be deposited during the active phases of deposition and finally repeating, if necessary, the last step by setting up one or more cathode(s) made of another metal and readjusting the cathode potential and the anode/cathode potential difference to new values depending on the new type of metal to be recovered.
Secondly, the invention relates to a process for the treatment and recycling of at least some effluents disposed of by photographic development laboratories and/or workshops, in particular waste photographic baths from fixing or bleaching and fixing, characterised in that it involves passing said effluents, in particular photographic baths and/or film washing water over a cationic ion-exchange resin which is selective relative to silver and capable of fixing it in the form of new complexes different from those present in said effluents, then regenerating said resin by elution of the silver in a regenerating solution containing a ligand capable of separating the silver from the resin and of binding it by complexing and, finally, carrying out, successively or simultaneously with regeneration, electrolysis of the loaded regenerating solution so as to recover the silver in metallic form.
Finally, the invention also relates to an installation for the treatment and recycling of fixing or bleaching and fixing baths and optionally photographic washing water intended, in particular, for carrying out the aforementioned treatment and recycling process characterised in that it consists mainly of at least one exchanger connected to a tank for waste bath to be treated, to a treated bath tank, to a regenerating solution tank and to at least one voltameter respectively, via corresponding pipes equipped with shut-off members and, if necessary, means for the active transfer of appropriate liquids, all of said members and transfer means being monitored and controlled by a central control unit.